1. Field of the Invention
The present invention relates to a polarization element, and more particularly to an optical device including the polarization element such as an image pickup optical system, a projection display device (projector), an image processing apparatus, or a semiconductor manufacturing apparatus.
2. Related Background Art
Up to now, a polarization splitting element using, for example, a dielectric multi-layer film has been known. As shown in FIG. 37, the polarization splitting element allows P-polarized light 18 incident on multi-layer film 17 to transmit through the film at a Brewster's angle as light 19 shown in FIG. 37, and reflects S-polarized light 20 by interference of the multi-layer film as light 21.
The multi-layer film is constructed by stacking dielectric layers having different refractive indices. Assume that a layer having a high refractive index nH is referred to as a layer H and a layer having a refractive index nL lower than the refractive index nH is referred to as a layer L. In general, a Brewster's angle θB between two media having refractive indices n1 and n2 is expressed by the expression (1). Of incident light beams at this angle, a P-polarized light component passes through all the dielectric layers.tan θB=n2/n1  (1)
In order to realize the polarization splitting element, it is necessary to establish a relationship between the refractive indices and the angle, in both a prism medium and an interface between the layer H and the layer L. Therefore, it is necessary to satisfy the following relational expression (2) between a refractive index np of the prism medium and refractive indices nH and nL of two dielectric media composing a thin film.
                              n          p                =                                                            n                H                2                            ⁢                              n                L                2                                                                    sin                2                            ⁢                                                θ                  B                                ⁡                                  (                                                            n                      H                      2                                        ⁢                                          n                      L                      2                                                        )                                                                                        (        2        )            
With respect to the S-polarized light, a reflective film by virtue of multi-layer film interference is realized using reflection on the interface due to the refractive index difference between the refractive indices nH and nL of a high refractive index medium and a low refractive index medium, respectively. A film thickness of each of the layers is optimized and 20 to 40 layers are stacked. Therefore, it is possible to realize a reflective film that causes reflection over the entire visible light region. With respect to the S-polarized light, a wide-angle characteristic and a wide-wavelength characteristic can be designed by increasing the number of layers of the film. On the other hand, transmittance with respect to the P-polarized light depends on the refractive indices between the media and the incident angle, so that the transmittance does not depend on a change in film thickness. The more the number of layers increases, the more reflectance with respect to the P-polarized light due to a deviation from the Brewster's angle increases. Therefore, wavelength and angle characteristics of the transmittance deteriorate.
A polarization splitting element in which a birefringent adhesive is sandwiched between prisms as described in U.S. Pat. No. 5,042,925 has been known as a polarization splitting element that does not use the multi-layer film. This uses a difference between refractive indices for an ordinary ray and an extraordinary ray of the birefringent material. Although the difference of refractive indices therebetween is small, a large incident angle of about 60° is set to totally reflect one of the polarized light beams in a selective manner, thereby realizing polarization splitting.
When the total reflection is to be caused, it is necessary that the incident angle be equal to or larger than a critical angle θc. The critical angle θc is expressed by the following expression (3).sin θC=n2/n1  (3)
There has been known a polarization splitting element using birefringence in which a multi-layer film is etched to obtain a one-dimensional grating as shown in FIG. 36. The multi-layer film in which layers H 15 such as TiO2 layers and layers L 16 such as SiO2 layers are alternately stacked is etched to obtain the one-dimensional grating. When a period of the grating is made equal to or shorter than a wavelength of used light, the grating exhibits a birefringent characteristic with respect to incident light.
Such a birefringent characteristic caused depending on the structure of matter is called structural birefringence. The polarization splitting element can be realized by combining materials of the multi-layer film, and suitably setting a grating shape. In this specification, a structure having a period shorter than a wavelength λ of the used light, such as the one-dimensional grating, is referred to as a sub-wavelength structure (SWS).
The used light in this specification indicates light having a wavelength range corresponding to an optical element to be used. For example, assume that light beam from a light source which is made incident on an optical element to be used for visible light has a wide wavelength band, more specifically, includes light other than the visible light, such as ultraviolet light or infrared light in addition to the visible light. In this case, the light other than the visible light is also made incident on the optical element. Even in such a state, assume that the used light for the optical element to be used for visible light is visible light. The visible light is light having a wavelength of within a range of about 400 nm to 700 nm.
A refractive index of the SWS grating can be treated as an effective refractive index. In a grating as shown in FIG. 10A, assume that polarized light in a periodic direction of the grating is TM polarized light and polarized light in a direction orthogonal to the periodic direction is TE polarized light. Here, there has been known that effective refractive indices nTE and nTM with respect to the respective polarized lights in a one-dimensional grating in which media having refractive indices, n1 and n2, are repeated at a width ratio of a:b, are generally expressed by the expressions (4) and (5).
                              TE          ⁢                                          ⁢                      n            TE                          =                                                            an                1                2                            +                              bn                2                2                                                    a              +              b                                                          (        4        )                                          TM          ⁢                                          ⁢                      n            TM                          =                                            a              +              b                                                      a                /                                  n                  1                  2                                            +                              b                /                                  n                  2                  2                                                                                        (        5        )            Here, nTE>nTM is satisfied regardless of the ratio of a:b.
In the one-dimensional grating, assume that the medium of n1 is a dielectric and the medium of n2 is air. When a ratio of a dielectric width to a grating pitch is set as a filling factor f, the filling factor f is expressed by the expression (6). In this example, etching is performed such that the filling factor becomes about 0.5.f=a/(a+b)  (6)
FIG. 10B is a graph showing a change in effective refractive index relative to the filling factor f of TiO2 in a grating in which the medium of n1 is TiO2 and the medium of n2 is air. Similarly, FIG. 10C is a graph showing a change in effective refractive index in a grating in which the medium of n1 is SiO2. As is apparent from the graphs, a difference between refractive indices of the layer H and the layer L in the TE direction is larger and a difference between refractive indices of the layer H and the layer L in the TM direction is smaller. When a suitable prism medium is used, a Brewster's angle condition is satisfied in the TM direction, with the result that the grating can transmit the P-polarized light. A thickness of each of the layers is independent of the Brewster's angle condition. When a film thickness of each of the layer H and the layer L is optimized, it is possible to form the dielectric multi-layer film. As a result, the dielectric multi-layer film is provided with a function for reflecting the S-polarized light, which means that a function of the polarization splitting element is obtained. This improves the degree of freedom of selection of media satisfying the Brewster's angle condition with respect to the P-polarized light as compared with the case of the polarization splitting element composed of only the dielectric thin film. Therefore, it is possible to simultaneously increase the reflectance for the S-polarized light. Thus, the polarization splitting element covering the entire visible light region can be composed of about 20 layers.
However, in the case of the polarization splitting element using the dielectric multi-layer film, the Brewster's angle condition is used to transmit the P-polarized light. Therefore, the refractive index of a prism glass material and the refractive index of thin film medium are limited by the expression (1), so it is hard to widen an incident angle characteristic. The incident angle characteristic cannot be widened even if the number of layer is increased.
In the case of the polarization splitting element in which the birefringent adhesive is sandwiched between the prisms, since the difference between refractive indices of the ordinary ray and the extraordinary ray of the adhesive is not large, it is necessary to set the incident angle to about 60° or more for the total reflection. Therefore, applications of usable optical systems are limited. In addition, a high polymer material or the like is used for the adhesive, so the polarization splitting element is inferior in terms of heat resistance and light fastness.
The stacked type polarization splitting element having the rectangular grating using the SWS structure is complex, which increases a manufacturing cost thereof. In addition, the Brewster's angle condition is used to transmit the P-polarized light, so it is hard to widen the incident angle characteristic as in the case of the dielectric multi-layer film. In particular, as is apparent from the grating structure shown in FIG. 36, the difference between refractive indices of the TE direction and the TM direction reduces as the incident angle increases. Therefore, an increase in reflectance at an incident angle that exceeds the Brewster's angle is larger than that in the case using the dielectric thin film. As a result, there is a limitation on the incident angle characteristic.
In the case of a polarization splitting element used for a liquid crystal projector or the like, a wide wavelength range which covers the entire visible light region, and a small FNo. for obtaining brightness (that is, a wide angle characteristic) are required.